1. Field of the Invention
The present invention relates a lithographic apparatus and device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Typically, if a source of radiation has jitter, a pulsed beam may be generated before or after a desired moment in time. This can cause a patterned light beam, formed from the pulsed beam interacting with a patterning device, to pattern the substrate in front of or behind a desired target position. A synchronous scanning mirror SSM, which is synchronized to both a frequency of the pulsed beam and a scanning velocity of the substrate, has typically been used to compensate for such jitter. Through using the synchronous scanning mirror SSM, the patterned beam can be directed onto a scanning substrate supported by a scanning substrate table at a desired target position. However, for maskless lithography, the synchronous scanning mirror SSM has to scan at high frequency (e.g., 6 kHz), a scanning motion has to be a pure rotation (small amplitude, +/−1.2 mrad), and a rotation axis must lie on an active surface of the synchronous scanning mirror SSM. Also, the synchronous scanning mirror SSM has to be of an excellent optical quality (i.e., very flat, in the order of about 0.5 nm flatness), which requires it to have substantial thickness, resulting in a large mass.
Conventional synchronous scanning mirror SSMs are typically incapable of producing a pure rotational scanning motion with low mirror distortion. Dynamically, the unavoidable existence of undesired modes with low eigenfrequencies (e.g., one of the frequencies with which a particular system may vibrate) makes it impossible to achieve a pure, distortion-free rotation. A second mode having eigenfrequency only 2× higher than the useful first mode produces out-of-plane translation and mirror distortion. In addition, from a manufacturability point of view, the requirement that an axis of rotation lay on a reflective surface of the synchronous scanning mirror SSM makes it difficult to access the reflective surface for polishing, unless the reflective surface is a separate part. With a separate reflective surface, mounting the reflective surface without introducing distortion and without introducing modes lower than the very high scanning frequency can present an unsolvable compromise.
Therefore, what is needed is a system and method that allows for a more effective synchronous scanning mirror SSM.